Aqueous lime slurries have been used to remove sulfur dioxide from gaseous streams and still provide one of the major commercial systems for removing sulfur dioxide from hot gaseous streams, such as those produced in commercial power plants. Other popular basic flue gas desulfurization processes include limestone scrubbing, magnesium enhanced lime scrubbing, and dual alkali scrubbing processes, with various modifications of these basic processes in existence.
In the magnesium-enhanced lime scrubbing systems, a controlled amount of magnesium ion is provided in the aqueous lime scrubbing slurry, as described in U.S. Pat. No. 3,919,393, assigned to Dravo Lime Company, the contents of which are incorporated by reference herein. As described therein, about 2-10 percent by weight of magnesium oxide is added to the calcium oxide, which provides a magnesium ion content in a wet scrubbing unit so as to eliminate scale formation in the wet scrubbing unit while minimizing the amount of magnesium sulfite and sulfate in the effluent from the wet scrubbing unit While this process has found favor in the industry, a drawback exists in that the filter cake produced upon separating solids from the effluent from the wet scrubbing unit is relatively wet and contains typically only about 40-45% solids This gives rise to relatively high disposal costs and difficulty in handling of the wet solids or sludge, as it is called.
The dual alkali scrubbing process, which uses an alkali, such as sodium, in aqueous solution to remove sulfur dioxide from gaseous streams, while using a solution and thus avoids scaling in the wet scrubbing unit, have a relatively high cost connected therewith due to the soda ash (Na.sub.2 CO.sub.3) used to provide soluble sodium for scrubbing. The dual alkali process scrubs with a clear liquor and generally produces a relatively dry filter cake, but at the relatively high cost associated with the use of soda ash. Examples of various dual alkali processes are described in U.S. Pat. No. 3,607,033; U.S. Pat. No. 3,775,532; U.S. Pat. No. 3,873,532; U.S. Pat. No. 3,944,649; U.S. Pat. No. 3,989,464; U.S. Pat. No. 3,989,797; U.S. Pat. No. 4,147,756; and U.S. Pat. No. 4,411,875, the contents of these eight patents incorporated by reference herein.
In the dual alkali processes, the alkali metal compound, such as soda ash, in aqueous solution, is used in the wet scrubbing unit and sodium salts, such as sodium sulfite removed from a bleed stream from the wet scrubbing unit. The sodium component is regenerated, outside the wet scrubbing unit, such as by addition of lime to the effluent which reacts with the sodium sulfite to form calcium sulfite, that is removed from the system, and sodium sulfite which is returned to the wet scrubbing unit for further scrubbing of sulfur dioxide gases In such processes, it is emphasized that good separation of the regeneration products be effected so as to prevent return of calcium ions to the scrubber which could cause scaling
It is an object of the present invention to provide a sulfur dioxide scrubbing method using lime that decreases the high cost of soda ash associated with the dual alkali processes and produces a dry filter cake from the scrubber effluent while using the reliability of having soluble alkalinity associated with a magnesium-enhanced lime process for neutralizing absorbed sulfur dioxide.